This invention relates in general to audio systems and in particular to a novel arrangement for reducing distortion in multiple amplifier, multiple loudspeaker installations. The progress in digital recording has created an opportunity for advancing the accuracy of sound reproduction. The latest advances in digital to analog signal conversion yield low noise signal sources with dynamic ranges of up to 90 dB. When signals from such sources are reproduced through loudspeakers of improved resolution and dynamic accuracy such as those described in U.S. Pat. Nos. 4,597,100 and 5,070,530 and in co-pending U.S. application Ser. No. 995,833, the resulting high quality audio can be used to make more accurate listening evaluations. In particular, new insights into the reproduction of ambient information and of the accuracy of stereo imaging become possible. Using these improved playback resources in a variety of different sized rooms and spaces has uncovered previously overlooked problems in sound reproduction, especially where more than one loudspeaker is used as a sound source.
A first problem occurs in sound systems having multiple loudspeakers that are spaced some distance apart and results from a combination of sound transmission directly between the loudspeakers and the reflective interactions of the total sound field with the boundaries of the room. A pair of loudspeakers are the norm for stereo reproduction and multiple loudspeakers are also common in sound reinforcement (public address type) applications. These direct transmission and boundary reflection processes interact with the amplification system to undesirably reinforce room colorations, echoes and acoustic feedback.
A second problem occurs in most listening areas because the acoustic surrounding of each loudspeaker system, and thus the acoustic loading on each loudspeaker, is different. As a result of this loading difference, the acoustic output of each of the loudspeakers is not identical. Loading differences, even when small, have been found to degrade reproduction of stereo "phantom" center images and cause the loudspeaker placement in the listening area to be critical.
This invention discloses a circuit that is connected between the four output terminals of a pair of power amplifiers to reduce the effects of loudspeaker-to-loudspeaker coupling and loudspeaker-to-room interaction. In stereo sound reproduction systems this invention results in improvements in harmonic accuracy, stereo imaging and bass reproduction. In sound reinforcement systems, the invention reduces acoustic feedback and greatly improves intelligibility.